The present invention generally relates to servo information reproducing methods and magnetic disk units, and more particularly to a servo information reproducing method for accurately reproducing servo information and to a magnetic disk unit which employs such a servo information reproducing method.
In magnetic disk units, a recording density of a magnetic disk is improved by taking measures such as reducing a track pitch on the magnetic disk. Servo information that is used to detect a position on the magnetic disk is recorded on the magnetic disk, and there are demands to accurately reproduce the servo information even when the track pitch is reduced.
FIG. 1 is a diagram showing a magnetic disk 100 having a recording surface on which servo regions 101 recorded with servo information and data regions 102 recorded with data coexist. For example, each cylinder (or track) on the recording surface of the magnetic disk 100 is provided with 50 to 100 servo regions 101.
FIG. 2 is a diagram showing a track pattern for a case where each track is provided with 60 servo regions 101-1 through 101-60, for example. In FIG. 2, when the rotational speed of the magnetic disk 100 is 5400 rpm, each of the servo regions 101-1 through 101-60 are 30 .mu.sec, for example, and 1 track is approximately 11.1 msec.
In the data region 102 shown in FIG. 1, the data is recorded with a recording format shown in FIG. 3. In FIG. 3 and FIG. 4 which will be described later, R denotes a radial direction of the magnetic disk 100, and an arrow extending in a horizontal direction indicates the magnetization direction. Each of tracks T1 and T2 is recorded by a head having a write core width WW, and a dead space DS is formed between the two adjacent tracks T1 and T2.
On the other hand, in the servo region 101 shown in FIG. 1, the servo information is recorded with a recording format shown in FIG. 4. In other words, the servo information is recorded consecutively without forming a dead space between the adjacent tracks, so that it is possible to reproduce the servo information regardless of the position of the head on the magnetic disk 100. A head having a write core width WW wider than a servo track width SW is used to record the servo information, so that the write core width WW partially overlaps when recording the adjacent servo tracks. That is, an overwrite portion OVR is formed as shown in FIG. 5 because the adjacent servo tracks are formed while partially overwriting the previously formed one of the adjacent servo tracks. A head having a read core width RW which is narrower than the write core width WW is used when reproducing the servo information.
In addition, when recording the servo information by the head, information erasure occurs on both sides of the head due to the characteristic of the head. This information erasure is the so-called side erase. This side erase does not become a problem in the data region 102 where the dead space DS is formed between the adjacent tracks T1 and T2. However, since no dead space is formed in the servo region 101, the servo information recorded in the servo region 101 is erased by a side erase SE at one side of the overwrite portion OVR as shown in FIG. 5.
Conventionally, the read core width RW is considerably wider compared to the width of the side erase SE, that is, RW&gt;&gt;SE. For this reason, a decrease in the reproduced output level of the servo information caused by the side erase SE is on the order of approximately 10%, for example, and no serious problem is introduced by the side erase SE.
But as the recording density of the magnetic disk 100 increases and the track pitch becomes extremely small, the relationship between the read core width RW and the width of the side erase SE becomes RW&gt;SE, and the ratio of the width of the side erase SE with respect to the read core width RW increases. As a result, there was a problem in that the effects of the side erase SE on the reproduced output level of the servo information can no longer be neglected.
More particularly, when the track pitch becomes small, the write core width WW and the read core width RW become narrow. On the other hand, the width of the side erase SE is independent of the write core width WW, and is approximately 0.2 .mu.m, for example, and is substantially constant. FIG. 6 is a diagram for explaining a case where the side erase SE exists in the servo region 101. In this case, when the data region 102 is reproduced by the read core width RW as indicated by the hatching in FIG. 6, it is possible to satisfactorily reproduce the data because the data is recorded with the write core width WW which is wider than the read core width RW. On the other hand, in the servo region 101, the side erase SE is generated at positions shown in FIG. 6 due to the overwriting described above. Hence, when the servo region 101 is reproduced by the read core width RW, the side erase SE occupies a relatively large portion of the reproduced part, and the reproduced output level of the servo information greatly decreases. For example, the decrease of the reproduced output level of the servo information is approximately 30%. A reproduced output SOE of the servo information for the case where the side erase SE exists can be described by SOE.apprxeq.{(RW-SE)/RW}.multidot.SO, where SO denotes a reproduced output of the servo information for a case where no side erase SE exists, and SE denotes the width of the side erase SE.
When the reproduced output level of the servo information greatly decreases, there were problems in that it is impossible to accurately reproduce the servo information, and that it is impossible to accurately carry out a position control of the head with respect to the magnetic disk 100.